1. Field of the Invention
This invention relates to the processing of material in a fluidised bed, and in particular to a method of operating a fluidised bed for use in processing an article which is immersed in the bed.
More particularly the invention relates to a method and apparatus for thermally treating a glass article, for example a glass sheet, in which the article is immersed in a gas-fluidised bed of particulate material which is maintained in a quiescent uniformly expanded state of particulate fluidisation. The thermal treatment of the glass sheet may be for annealing, thermally toughening or heating the sheet.
The invention is particularly applicable to a method and apparatus for thermally toughening a succession of flat glass sheets, or bent glass sheets such as those used singly as a motor vehicle windscreen or as part of a laminated motor vehicle windscreen, a side light or a rear light for a motor vehicle, or for use in construction of windscreen assemblies for aircraft and railway locomotives.
2. Description of the Prior Art
In U.S. patent application Ser. No. 717171, filed 24th Aug. 1976, now abandoned the disclosure of which is incorporated herein by reference, there is described a method of thermally treating glass in which the hot glass is immersed in a fluidised bed of particulate material. Prior to immersion of the glass the fluidised bed is in a quiescent uniformly expanded state of particulate fluidisation. By using this method a successful commercial yield of whole glass sheets is achieved, there being very few fractures of the glass sheets while the toughening stresses are being developed in the sheets.
When thermally toughening a succession of glass sheets, which may be flat or bent, each sheet is previously heated in known manner to a temperature above the strain point of the glass and usually to a temperature near to the softening point of the glass, for example a temperature in the range 610.degree. C. to 670.degree. C. when toughening soda-lime-silica glass. Each hot glass sheet is suspended from tongs and is immersed in the fluidised bed and toughening stresses are induced in the glass as it cools in the fluidised bed. In order to achieve a consistent degree of toughening of a succession of glass sheets it is desirable to maintain the fluidised bed at a constant temperature, for example at a temperature in the range 40.degree. C. to 150.degree. C., and more usually in the range 40.degree. C. to 70.degree. C.
Further in commercial operation it is desirable to process a succession of glass sheets in rapid sequence and the thermal toughening of one glass sheet every 60 seconds is typical. This means that there is a large input of heat to the particulate material of the fluidised bed.
It has already been proposed to cool a fluidised bed of the freely bubbling type used for the processing of glass sheets, by the provision of water cooled coils immersed in the bed. Such coils are effective in removing heat from the fluidised bed because of the high rapidity of particulate movement and the high degree of particle mixing which occurs in such a freely bubbling fluidised bed.
We have found that the use of such cooling coils is not sufficiently effective for regulating the temperature of a fluidised bed of particulate material which is maintained in a quiescent uniformly expanded state of particulate fluidisation, and which is being used for the processing of a succession of glass articles.
In the case of such a fluidised bed for use in the thermal treatment of, for example, glass sheets, the cooling coils have to be placed at positions where they will not interfere with the entry of glass sheets into the bed, for instance around the sides of the bed. This places a limitation on the number of cooling coils which can be fitted in the bed and thus limits the degree of cooling which can be achieved.
It is a main object of the present invention to provide an improved method and apparatus for operating a fluidised bed, in particular for regulating the temperature of a fluidised bed which is in a quiescent uniformly expanded state of particulate fluidisation.